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Date:   Fri, 28 Jun 2019 05:27:35 -0700
From:   "Paul E. McKenney" <paulmck@linux.ibm.com>
To:     Byungchul Park <byungchul.park@lge.com>
Cc:     Scott Wood <swood@redhat.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        rcu <rcu@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>
Subject: Re: [RFC] Deadlock via recursive wakeup via RCU with threadirqs
Reply-To: paulmck@linux.ibm.com
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 <20190627173831.GW26519@linux.ibm.com>
 <20190627181638.GA209455@google.com>
 <20190627184107.GA26519@linux.ibm.com>
 <13761fee4b71cc004ad0d6709875ce917ff28fce.camel@redhat.com>
 <20190627203612.GD26519@linux.ibm.com>
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On Fri, Jun 28, 2019 at 07:40:45PM +0900, Byungchul Park wrote:
> On Fri, Jun 28, 2019 at 04:31:38PM +0900, Byungchul Park wrote:
> > On Thu, Jun 27, 2019 at 01:36:12PM -0700, Paul E. McKenney wrote:
> > > On Thu, Jun 27, 2019 at 03:17:27PM -0500, Scott Wood wrote:
> > > > On Thu, 2019-06-27 at 11:41 -0700, Paul E. McKenney wrote:
> > > > > On Thu, Jun 27, 2019 at 02:16:38PM -0400, Joel Fernandes wrote:
> > > > > > 
> > > > > > I think the fix should be to prevent the wake-up not based on whether we
> > > > > > are
> > > > > > in hard/soft-interrupt mode but that we are doing the rcu_read_unlock()
> > > > > > from
> > > > > > a scheduler path (if we can detect that)
> > > > > 
> > > > > Or just don't do the wakeup at all, if it comes to that.  I don't know
> > > > > of any way to determine whether rcu_read_unlock() is being called from
> > > > > the scheduler, but it has been some time since I asked Peter Zijlstra
> > > > > about that.
> > > > > 
> > > > > Of course, unconditionally refusing to do the wakeup might not be happy
> > > > > thing for NO_HZ_FULL kernels that don't implement IRQ work.
> > > > 
> > > > Couldn't smp_send_reschedule() be used instead?
> > > 
> > > Good point.  If current -rcu doesn't fix things for Sebastian's case,
> > > that would be well worth looking at.  But there must be some reason
> > > why Peter Zijlstra didn't suggest it when he instead suggested using
> > > the IRQ work approach.
> > > 
> > > Peter, thoughts?
> > 
> > Hello,
> > 
> > Isn't the following scenario possible?
> > 
> > The original code
> > -----------------
> > rcu_read_lock();
> > ...
> > /* Experdite */
> > WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
> > ...
> > __rcu_read_unlock();
> > 	if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
> > 		rcu_read_unlock_special(t);
> > 			WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
> > 			rcu_preempt_deferred_qs_irqrestore(t, flags);
> > 		barrier();  /* ->rcu_read_unlock_special load before assign */
> > 		t->rcu_read_lock_nesting = 0;
> > 
> > The reordered code by machine
> > -----------------------------
> > rcu_read_lock();
> > ...
> > /* Experdite */
> > WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
> > ...
> > __rcu_read_unlock();
> > 	if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
> > 		rcu_read_unlock_special(t);
> > 		t->rcu_read_lock_nesting = 0; <--- LOOK AT THIS!!!
> > 			WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
> > 			rcu_preempt_deferred_qs_irqrestore(t, flags);
> > 		barrier();  /* ->rcu_read_unlock_special load before assign */
> > 
> > An interrupt happens
> > --------------------
> > rcu_read_lock();
> > ...
> > /* Experdite */
> > WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
> > ...
> > __rcu_read_unlock();
> > 	if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
> > 		rcu_read_unlock_special(t);
> > 		t->rcu_read_lock_nesting = 0; <--- LOOK AT THIS!!!
> > <--- Handle an (any) irq
> > 	rcu_read_lock();
> > 	/* This call should be skipped */
> > 	rcu_read_unlock_special(t);
> 
> Wait.. I got a little bit confused on recordering.
> 
> This 'STORE rcu_read_lock_nesting = 0' can happen before
> 'STORE rcu_read_unlock_special.b.exp_hint = false' regardless of the
> order a compiler generated to by the barrier(), because anyway they
> are independent so it's within an arch's right.
> 
> Then.. is this scenario possible? Or all archs properly deal with
> interrupts across this kind of reordering?

Interrupts are "exact" in that they happen between a pair of consecutive
instructions from the viewpoint of the CPU taking the interrupt.  And
again, these fields are local to their CPU/task, give or take debug code.

							Thanx, Paul

> Thanks,
> Byungchul
> 
> > 			WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
> > 			rcu_preempt_deferred_qs_irqrestore(t, flags);
> > 		barrier();  /* ->rcu_read_unlock_special load before assign */
> > 
> > We don't have to handle the special thing twice like this which is one
> > reason to cause the problem even though another problem is of course to
> > call ttwu w/o being aware it's within a context holding pi lock.
> > 
> > Apart from the discussion about how to avoid ttwu in an improper
> > condition, I think the following is necessary. I may have something
> > missing. It would be appreciated if you let me know in case I'm wrong.
> > 
> > Anyway, logically I think we should prevent reordering between
> > t->rcu_read_lock_nesting and t->rcu_read_unlock_special.b.exp_hint not
> > only by compiler but also by machine like the below.
> > 
> > Do I miss something?
> > 
> > Thanks,
> > Byungchul
> > 
> > ---8<---
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 3c8444e..9b137f1 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -412,7 +412,13 @@ void __rcu_read_unlock(void)
> >  		barrier();  /* assign before ->rcu_read_unlock_special load */
> >  		if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
> >  			rcu_read_unlock_special(t);
> > -		barrier();  /* ->rcu_read_unlock_special load before assign */
> > +		/*
> > +		 * Prevent reordering between clearing
> > +		 * t->rcu_reak_unlock_special in
> > +		 * rcu_read_unlock_special() and the following
> > +		 * assignment to t->rcu_read_lock_nesting.
> > +		 */
> > +		smp_wmb();
> >  		t->rcu_read_lock_nesting = 0;
> >  	}
> >  	if (IS_ENABLED(CONFIG_PROVE_LOCKING)) {
> > 
> > 
>